1. Field of the Invention
The invention relates to a transmission circuit for a telephone set, comprising a tone generator for generating two groups of dialling tones, the tones of the first group all having a lower frequency than the tones of the second group, having directly connected thereto a passive first-order low-pass tone filter, the transmission circuit comprising a microphone amplifier for amplifying the speech signal and a passive first-order low-pass speech filter for limiting the frequency band of the speech signal, the transmission circuit further comprising a switch for selectively connecting the input of the speech filter to the output of the tone filter or of the microphone amplifier.
2. Description of the Related Art
Such a transmission circuit is disclosed in the periodical "Electronic Components and Applications", February 1982, pages 89 to 99.
A system of dialling in which the number to be dialled is represented by a signal formed by a tone from the first group and a tone from the second group is known as a DTMF-dialling system.
In order to satisfy the CCITT and CEPT requirements as regards the attenuation of the higher harmonics produced during the generation of the tones, DTMF-transmission circuits must be provided with a second or higher-order filter arranged in series with the tone generator generating the tones. In prior art transmission circuits it is customary to choose a second-order active Butterworth filter, for example a Sallen & Key filter for this purpose.
To obtain the required frequency dependence of the filter attenuation it is necessary that the amplifier of the active filter has a low output impedance at all frequencies. At high frequencies this is only possible with a comparatively high current consumption of the amplifier, which is disadvantageous, particularly for an amplifier in an integrated circuit. In addition, when the transmission circuit is constructed in integrated circuit form the integrated circuit requires an additional connecting pin for connecting the external filter components to this amplifier, which is also a disadvantage.
The transmission circuit disclosed in the said Periodical "Electronic Components and Applications" does not comprise an active second-order filter, so that this circuit does not have the two above-mentioned disadvantages. In the transmission circuit described in this periodical, filtering of the dialling tones is effected with the aid of a second-order passive filter which is formed by a series arrangement of a first-order passive tone filter and a first-order passive speech filter provided for filtering the microphone signal, which series arrangement is exclusively used during dialling. This mode of filtering does not require a separate second-order tone filter in addition to the speech filter. Now, instead of a second-order tone filter the first-order tone filter is sufficient, which is advantageous more specifically when integrated circuits are used.
When assembling the second-order tone filter from the two first-order filters the problem is that the speech filter has a cut-off frequency which is considerably higher than the cut-off frequency of the second-order tone filter, which latter cut-off frequency is desired for attenuating the higher harmonics. In order to realize the desired second-order cut-off frequency all the same, the first-order tone filter must have a considerably lower cut-off frequency than the desired second-order cut-off frequency. Because of this comparatively low cut-off frequency of the first-order tone filter the dialling tones do however no longer satisfy the CCITT and CEPT requirements as regards their levels.